As a secondary battery for portable devices, a lithium ion secondary battery is widely used. For example, a lithium metal oxide such as LiCoO2 or LiMn2O4 is used for a positive electrode of the lithium ion secondary battery, while a carbonaceous material absorbing and releasing lithium ions is used for a negative electrode.
On the other hand, there are demands for the secondary battery used for vehicles or electricity storage systems, such as storage performance, float charge resistance, cycle life performance, high output, safety, and long-term reliability in high temperature environments. Accordingly, a material excellent in chemical and electrochemical stability is required as a material of the positive and negative electrodes of the secondary battery. Study of LiFePO4 as a positive-electrode material is proceeded; however, there are problems such as a decrease in energy density due to a decrease in electromotive force and a decrease in high-temperature endurance. Therefore, in order to use the secondary battery (which is mounted on an electricity storage system to be fixed) as a power source in place of a lead storage battery, there is a need for a secondary battery excellent in energy density, cycle life, and float charge resistance.